Heat-generating magnetic ink compositions



United States Patent 3,360,466 HEAT-GENERATING MAGNETIC INK COMPOSITIONSOliver A. Short, Wilmington, Del., assignor to E. I. du Pont de Nemoursand Company, Wilmington, Del., a corporation of Delaware No Drawing.Filed Nov. 18, 1964, Ser. No. 412,253 4 Claims. (Cl. 252-62.S3)

ABSTRACT OF THE DISCLOSURE A heat-generating magnetic lithographic typeink consisting essentially of a finely divided magnetic pigmentpossessing a high energy loss and a coercivity of 2 to 20 oersteds, anda liquid vehicle having drying properties and-,containing a metalcompound drier. Since the ink heats'instantly to a given hightemperature when subjected to amagnetic field, it is particularly usefulas a source of sudden high heat, e.g., when a print thereof under aheat-scalable coating on paper or the like is subjected to a magneticfield.

Wax and wax-containing coatings have long been applied to paper,paperboard and similar structures to permit the heat-sealing of suchstructures to each other. The heat for such heat-sealing has usuallybeen supplied by the hot surface of an electric heating element, gasflames, hot air blasts and the. like, the heat-seal being effected bycontacting the hot surface of the heating element or the gas flame withone or both sides of the portions of the coated structures to be sealed.

It was found recently that the heating of such heat-sealable coatingscan be greatly speeded up by applying a finely divided magneticallyactuable substance to the coatings and subjecting the finely dividedsubstance to a magnetic field to quickly heat and melt the coatingswhereby to heat-seal the coated structures. Application of suchmagnetically actuable substance would be most practical if it could beemployed in an ink which could be rapidly applied employing usualprinting methods and equipment.

It is an object of the invention to provide a new and improvedheat-generating magnetic ink. A particular object is to provide amagnetic ink that can be printed at high speeds, for example, in alithographic or raised letter press, as an undercoat under aheat-scalable coating on paper or similar material, and has the propertyof instantaneously heating to a given temperature, when subjected to amagnetic field, to cause the heat-scalable over coating to heat-seal.Other objects of the invention Will be apparent from the followingdescription.

The ink of the invention consists essentially of a finely dividedmagnetic pigment having a low coercivity but a high energy loss and aviscous tacky vehicle therefor comprising a drying liquid component anda metal compound drier dissolved therein. The weight ratio of the abovepigment to the vehicle should be between 0.9:1 to 4:1, the preferredratios ranging from 1:1 to 3.5: 1.

The pigment, which should be in finely divided form, e.g., have anaverage particle size of 0.5 to microns in diameter, must have a lowcoercivity. Pigments with a coercivity of from 2 to 20 oersteds aregenerally suitable. The finely divided metal ferrites containing, on anoxide weight basis, 3 to 30% NiO, 2 to 20% ZnO, 0 to 5% MnO and at least66% (generally the remainder) Fe O are magnetic pigments meeting theabove requirements. Such magnetic materials which are availablecommercially as sintered shapes which must be crushed and ground to makepigments, are different in kind from those commonly used on tapes, bankchecks and the like where a high remnant magnetism is essential and ahigh coercivity is desirable. This latter type of magnetic pigment isentirely unsuitable for use in formulating the ink of the presentinvention.

The ink should possess the property of drying when the print thereof onpaper, paperboard or the like is exposed to air. It should also bedefinitely tacky and adhesive and have a viscosity of 50,000 to 100,000centipoises in order to permit printing at high speeds in a lithographicor raised letterhead press. In order that these requirements be met, thevehicle for the magnetic pigment particles must include a liquidcomponent having drying properties and a metal compound drier, such asthe cobalt, manganese, cerium and other metal acetates, borates,oxalates, linoleates, linoleneates, naphthenates, xanthates andZ-ethylhexanates employed as driers for the oxidative drying of linseedoil, eg, in lithographic varnishes, Such driers should be present inamounts, e. g., from 0.2 toabout 3%, sufficient to effect the desireddrying of the printed hyde resins, rosin-modified maleic ester resinsand alkyd resins having long chain unsaturated groups, such as thestyrenated linseed or tung oil alkyds. If a non-drying resin is used, asolution thereof in a drying oil such as linseed oil or linseed standoil should be used as the liquid drying component of the vehicle. If aresin having drying properties is used, it may also be employed as asolution in such a solvent, although non-drying solvents such as theali-.

phatic hydrocarbons boiling at from about 240 to 300 C. can be used.Thus, the drying property of the ink may derive from the use of a dryingresin or a drying oil, which drying substance should be so selected asto provide an ink which is tacky and has a viscosity within the rangestated above. Minor adjustment of the viscosity to bring it within thisrange may be effected, when necessary, by thinning With a hydrocarbonsolvent of the kind indicated above.

The ink of the invention adheres well both to substrates such as paperand paperboard and to heat-sealable wax and wax-containing coatings.When formulated to contain from 0.9 to 4 parts of the magnetic pigmentper one part of the vehicle, prints thereof can be rapidly andeffectively applied by lithographic or raised letter presses. The aboveweight ratio of pigment to vehicle is important since if less than 0.9part of pigment is used per part of vehicle the print will not containsufiicient pigment to effect the desired heat sealing unless multipleprints are used. On the other hand, if more than 4 parts of pigment areemployed per part of vehicle, bonding of the pigment to the papersubstrate will be inadequate.

The invention is illustrated by the following examples.

Example 1 A magnetic pigment was prepared by ball milling a crushedferrite formed by sintering together NiO, ZnO and Fe O in the weightratio of 10 NiO:5 ZnO: Fe O This pigment had a coercivity of about 2oersteds. The ball milling was effected in a porcelain mill withporcelain balls in water for about 16 hours, whereby the averageparticle size was reduced to about 7 microns. The pigment was mixed witha commercial heat-bodied linseed oil based varnish containing a drier ata Weight ratio of one part of the pigment to one part of the varnish.The pigment-varnish mixture was thoroughly blended by passing it througha 3-roll paint or ink mill. The ink thus formed has a viscosity of50,000 centipoises (Brookfield Vis- Patented Dec. 26, 1967 cometer RVTNo. 6 Spindle at r.p.m.). It was printed by means of a lithographicoffset press onto a paperboard typical of that used for frozen foodcartons, Carton blanks were then cut from this board and coated with anethylene/ vinyl acetate copolymer-wax blend. When two pieces of thiscoated board were held together in an 80 mega cycle magnetic field, asatisfactory seal was formed in 150 milliseconds in those areas wherethe ink print underlay the copolymer-wax blend coating.

Example 2 An ink generally similar to that of Example 1 was formulatedusing 3 parts by weight of the magnetic pigment of Example 1 to one partby weight of the varnish as in Example 1 and with sufiicient petroleumspirits thinner to give an ink having a viscosity of 80,000 centipoises(Brookfield Viscometer RVT with No. 6 spindle at 10 rpm). This ink wasprinted on paperboard which was then coated with an ethylene/ vinylacetate copo-lymer-wax blend as described in Example 1. The paperboardwas cut and formed into food cartons and sealing was obtained in about50 milliseconds when the inked and coated surface was placed in an 80megacycle magnetic field against another coated surface which had notbeen inked.

The finely divided ferrite pigments employed in the ink of the inventionhave the property of heating substantially instantaneously to atemperature of 200 to 600 C. in a magnetic field of 200 oersteds or lessand between 30 and 3000 megacycles. Upon removalor discontinuance of themagnetic field, the ferrite particles of the printed ink return tonormal temperature, thus permitting the coating to congeal. The uppertemperature reached when the magnetic field is applied is self-limiting,since the ferrite particles on reaching their specific Nel temperature,i.e., that temperature at which the magnetic hysteresis effects cease,will heat no further. Thus, the ink of the invention can be employed toeffect substantially instantaneous sealing at temperatures which willnever be too high. This feature together with the fact that the ink canbe rapidly and effectively printed by lithographic or'raised letterpresses makes it highly practical for the purposes indicated.

The embodiments of the invention in which an eXclusive property orprivilege is claimed are as follows.

I claim: 0

1. A heat-generating magnetic ink consisting essentially, on a weightbasis, of 0.9 to 4 parts of a finely divided magnetic pigment possessinga high energy loss and a coercivity of 2 to 20 oersteds per 1 part ofa'liquid vehicle having drying properties and containing a metalcompound drier dissolved therein, said pigment being a metal ferritecontaining, on an oxide basis, 3 to 30% NiO, 2 to 20% 2110, 0 to 5% MnOand at least 66% Fe O 2. A magnetic ink according to claim 1 in whichthe vehicle consists essentially of heat-bodied linseed oil and a metalcompound drier and the ink has a viscosity of 50,000 to 100,000centipoises.

3. A heat-generating magnetic ink having a viscosity of 50,000 to100,000 centipoises and consisting essentially, on a weight basis, of(a) a finely divided magnetic ferrite pigment containing, on an oxideWeight basis, 3 to 30% NiO, 2 to 20% ZnO, 0 to 5% MnO and at least 66%Fe O and (b) a liquid vehicle having drying properties and consistingessentially of a heat-bodied linseed oil having dissolved therein ametal compound-drier.

4. A magnetic ink according to claim 3 wherein the ferrite pigmentcontains about 10% NiO, 5% ZnO and Fe O References Cited UNITED STATESPATENTS 3,082,171 3/1963 Shoemaker et al. 252-625 TOBIAS E. LEVOW,Primary Examiner.

R. D. EDMONDS, Assistant Examiner.

1. A HEAT-GENERATING MAGNETIC INK CONSISTING ESSENTIALLY, ON A WEIGHTBASIS, OF 0.9 TO 4 PARTS OF A FINELY DIVEDED MAGNETIC PIGMENT POSSESSINGA HIGH ENERGY LOSS AND A COERCIVITY OF 2 TO 20 OERSTEDS PER 1 PART OF ALIQUID VEHICLE HAVING DRYING PROPERTIES AND CONTAINING A METAL COMPOUNDDRIER DISSOLVED THEREIN, SAID PIGMENT BEING A METAL FERRITE CONTAINING,ON AN OXIDE BASIS, 3 TO 30% NIO, 2 TO 20% ZNO, 0 TO 5% MNO AND AT LEAST66% FE2O3.